This invention relates to a device adapted to be used in a horizontal boring and milling machine having a milling spindle and a boring bar that advances and retracts within the internal bore of the milling spindle.
During the boring operation of ordinary boring and milling machines, the boring bar has not been clamped to the milling spindle, but a braking force is applied to a feed screw of the boring bar for preventing movements of the boring bar relative to the milling spindle, thus presenting drawbacks of displacing the boring bar from the spindle and causing erroneous operations. The displacements of the boring bar are also caused by thermal expansions of the milling spindle and the boring bar, thus enhancing the erroneous operation, reducing the operational capability, and causing fretting corrosions of the machines. For preventing such displacements, a manual clamping device has been proposed but found to be extremely troublesome. An automatic clamping device of a construction comprising a resilient ring interposed between the outer surface of the boring bar and the inner surface of the milling spindle has also been proposed. The resilient ring has an annular recessed portion formed along the outer surface of the same. An oil passage provided with a plunger at an end thereof is extended through the milling spindle to be connected with the recessed portion. When the plunger is depressed for a predetermined distance, oil contained in the recessed portion and the oil passage is pressurized to deform the resilient ring so as to clamp the boring bar onto the milling spindle. The proposed device of the above described construction, however, exhibits a difficulty of substantially reducing the clamping force when air is mixed in the oil.
For eliminating such a difficulty, an improved construction having a fixed ring provided outwardly of the milling spindle has also been proposed. A gap is maintained between the outer surface of the milling spindle and the inner surface of the fixed ring, and an annular groove is formed on the internal surface of the fixed ring. When the boring bar is to be clamped, a pressurized oil is supplied through the fixed ring to the resilient ring provided between the milling spindle and the boring bar. Although this improved device can assure stable clamping of the boring bar, recent trend of increased diameter and rotating speed of the milling spindle inevitably increases the shearing resistance of oil flowing across the gap, thus increasing heat created in the gap, and causing a temperature rise in the milling spindle and the boring bar. Thermal expansion of the milling spindle and the boring bar decreases the precision of the cutting operation.